feat: support ReactiveCommand background execution (#473)

feat: support background ReactiveCommands

Implementation:
- add the RunInBackground ReactiveCommand attribute option
- emit CreateRunInBackground for synchronous command methods
- preserve task and observable command factories
- handle output scheduler overloads across ReactiveUI 23 and 24

Tests:
- add TUnit generator and integration coverage
- verify ReactiveUI 23, v24 Primitives, and v24 System.Reactive compatibility
- update generated command attribute snapshots

Documentation:
- document background command generation and supported method behavior

fix: restore Roslyn 4.8 band so .NET 8 SDK users work again

3.0.0 dropped the 4.8/4.12 analyzer bands, shipping only roslyn4.14 +
roslyn5.0. The compiler selects the highest analyzer band <= its own
Roslyn version, so every .NET 8 SDK (Roslyn 4.8 in 8.0.1xx through 4.11
in 8.0.4xx) found no matching band and the generators stopped loading;
the version-less code-fix assembly, built against 4.14, also tripped
CS9057. .NET 8 is supported through Nov 2026, so restore support:

- Re-add the generator's roslyn4.8 band, correctly pinned to
  Microsoft.CodeAnalysis 4.8.0 (the old Roslyn480 project had no
  VersionOverride and silently resolved to 4.14 under CPM). The 4.12+
  API surface stays compiled out via the absent ROSYLN_412 constant.
- Pin the version-less code-fix assembly to 4.8.0 as well. It loads on
  every host, and building against the oldest supported API keeps it
  forward-compatible (4.8 -> 5.0) with no CS9057, so -warnaserror .NET 8
  builds are unaffected.
- Pack three bands: roslyn4.8 (whole .NET 8 line + VS 17.12/17.13),
  roslyn4.14 (VS 17.14 / .NET 9), roslyn5.0 (VS 2026 / .NET 10).
- Drop the EOL net9 MAUI sample TFMs (net10 only).

Build + 891 tests green on net8/9/10; pack verified to contain
analyzers/dotnet/roslyn4.8/cs.

docs: correct minimum requirements — VS 2022 17.14 (Roslyn 4.14) and …

…ReactiveUI 23.2.28+

The 17.8.0 / ReactiveUI 19.5.31 minimums were stale: 3.0.0 drops the
Roslyn 4.8/4.12 bands, so VS 2022 17.14 is the floor; bump the documented
ReactiveUI minimum to the current release (23.2.28). Drop the obsolete
'older than 19.5.31' compatibility note.

Update release workflow to include new jobs and permissions

Update release workflow to include new jobs and permissions

housekeeping: Bump version from 2.5 to 2.6.x

chore: Bump version from 2.4 to 2.5.x

housekeeping: Update version to 2.4.x

housekeeping: Release 2.3.x

Resolve required keyword order for partial reactive properties. (#240)